A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissues. Some therapies include the delivery of electrical stimulation to such organs or tissues. Some medical devices may employ one or more elongated electrical leads carrying electrodes for the delivery of electrical stimulation to such organs or tissues, electrodes for sensing electrical signals within the patient, which may be generated by such organs or tissue, and/or other sensors for sensing physiological parameters of a patient.
Medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of electrical stimulation or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to a medical device housing, which may contain circuitry such as stimulation generation and/or sensing circuitry. In some cases, the medical leads and the medical device housing are implantable within the patient. Medical devices with a housing configured for implantation within the patient may be referred to as implantable medical devices.
Implantable cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical stimulation to the heart via electrodes carried by one or more implantable medical leads. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain normal sinus rhythm. For example, in some cases, an implantable medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting fibrillation.
Implantable medical leads typically include a lead body containing one or more elongated electrical conductors that extend through the lead body from a connector assembly provided at a proximal end of the lead end to one or more electrodes located at the distal lead end or elsewhere along the length of the lead body. The conductors connect to the stimulation and/or sensing circuitry within an associated implantable medical device housing to respective electrodes or sensors. Some electrodes may be used for both stimulation and sensing. Each electrical conductor is typically electrically isolated from other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Medical lead bodies implanted transvenously within the heart tend to be continuously flexed by the beating of the heart. Medical lead bodies implanted within the heart or outside of the heart (e.g., subcutaneously) may be subject to other stresses. For example, stress may be applied to the lead body, including the conductors therein, during implantation or lead repositioning. As another example, patient movement can cause the route traversed by the lead body to be constricted or otherwise altered, causing stresses on the lead body and conductors. Even without stresses, lead insulation materials or joints may degrade inside the human body. In rare instances, such stresses or degradations may cause an insulation breach of the lead body and, in some instances, externalization of one or more of the conductors of the lead.
Lead insulation breach and/or externalization of lead conductors as well as other lead related conditions may result in undesirable operation. In the case of cardiac leads, sensing of an intrinsic heart rhythm through a lead can be altered by lead related conditions. Identification of lead related conditions may allow modifications of the stimulation therapy or sensing, or lead replacement.